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In situ x-ray diffraction measurements and inverse Monte Carlo simulations of pair distribution functions were used to characterize the local structure of molten AuGa2 up to 16 GPa and 940 K. Our results document systematic changes in liquid structure due to a combination of bond compression and coordination increase. Empirical potential structure refinement shows the first-neighbor coordination of Ga around Au and of Au around Ga to increase from about 8 to 10 and 4 to 5, respectively between 0 and 16 GPa, and the inferred changes in liquid structure can explain the observed melting-point depression of AuGa2 up to 5 GPa. As intermetallic AuGa2 is an analogue for metallic SiO2 at much higher pressures, our results imply that structural changes documented for non-metallic silicate melts below 100 GPa are followed by additional coordination changes in the metallic state at pressures in the 0.2–1 TPa range achieved inside large planets.

The high-pressure behavior of HgO-montroydite was investigated up to 36.5 GPa using angle-dispersive X-ray diffraction. The tetragonal phase of this material (HgO-II), a distortion of the NaCl structure, transforms into the cubic NaCl structure (HgO-III) above ~31.5 GPa. The transformation of mercury oxide from the orthorhombic Pnma (HgO-I) structure to a tetragonal I4/mmm structure (HgO-II) is confirmed to occur at 13.5 ± 1.5 GPa. Neither of the high-pressure phases, HgO-II nor HgO-III, is quenchable in pressure. The derived isothermal bulk modulus of HgO-II and its pressure derivative strongly depend on the assumed zero-pressure volume of this phase, but our elasticity results on HgO-II nevertheless lie significantly closer to theoretical calculations than prior experimental results, and the measured pressure of the phase transformation to the NaCl structure is also in agreement with recent theoretical results. The general accord with theory supports the existence of significant relativistic effects on the high-pressure phase transitions of HgO.

We have carried out electronic structure calculations for iron under high pressure using pseudopotential plane wave methods. There is a controversy regarding the structure of iron at moderate pressures (30-100 GPa) and temperatures (1000-2400 K), with different experiments suggesting different structures, such as orthorhombic, double hexagonal close packed (dhcp), etc. Our earlier calculations using the linear muffin-tin orbital method within atomic-sphere-approximation had argued against the stability of the orthorhombic phase. The more accurate calculations presented here predict qualitatively the same results. We have additionally studied the stability of various phases of Fe at different compressions by calculating phonon frequencies. These rule out the stability of the orthorhombic phase. To validate our zero-temperature electronic-structure results at finite temperature, we have compared the shock Hugoniot and melting properties of iron with the results of our electronic structure calculations. Though we have used simplified models for these estimates, our predictions compare well with the experimental data. We thus propose that these models can be used to obtain the information about the high-pressure melting curves of planetary materials.

Laser driven shock wave transit time in thin aluminium targets was experimentally estimated by determining the shock emergence time at the rear of thin aluminium foils of varying thickness from 5 to 35 μm. A 20 J, 5 ns Nd:glass laser was focused to produce laser intensity of 1012 to 5 × 1013 W/cm2 on the targets which were placed in vacuum. Target foil movement was measured to an accuracy of 10 μm using optical shadowgraphy technique. This technique was used to accurately measure the shock transit time by recording the optical shadowgrams at various instants of time and thus identify the instant at which the foil is just set into motion. Shock transit time measured in foils of different thickness can give the value of shock velocity at a given laser intensity. Target motion recorded by shadowgraphy can also give the target foil velocity from which shock pressure can be estimated. Experimental values of shock transit time, shock velocity and shock pressure were observed to agree well with the values using one-dimensional multi-group radiation hydrodynamic simulations.

X-ray holography is a new method of structure determination based on measurement of interference of a known reference wave with an unknown object wave (containing information on atomic sites scattering the reference wave) so that phase information is preserved. Unlike X-ray diffraction, it does not demand for translational periodicity in the material. It is based on the idea similar to that of optical holography and has been tested on crystals, quasicrystals, thin films and doped semiconductors for their structure determination. In order to analyse potentials and limitations of this technique, we have carried out theoretical simulation studies on simple structures. In this paper we describe the basic algorithm of hologram generation and reconstruction of atomic positions from generated data. We illustrate this technique using Fe (bcc) single crystal as sample case to demonstrate its capabilities and limitations. Simulations were carried out on the Cu (fcc) structure and on complex structure such as the Al-Pd-Mn quasicrystal. Technical issues such as low signal to noise ratio, twin image problem etc have been discussed briefly to emphasize the need for high intensity X-ray source such as synchrotron for experiments and proper reconstruction algorithm. Finally the scope and potential of this technique have been discussed.

The results of electrical resistance measurements under pressure on single crystals of WSe2 are reported. Measurements up to 8.5 GPa are carried out using Bridgman anvil set up and beyond it using diamond anvil cell (DAC) up to a pressure of 27 GPa. There is no clear indication of any phase transition till the highest pressure is reached in these measurements.

We discuss the high pressure behaviour of zinc as an interesting example of controversy, and of extensive interplay between theory and experiment. We present its room temperature electronic structure calculations to study the temperature effect on the occurrence of its controversial axial ratio (c/a) anomaly under pressure, and the related electronic topological transition (ETT). We have employed a dense 63 X 63 X 29 k-point sampling of the Brillouin zone and find that the small (c/a) anomaly near 10 GPa pressure persists at room temperature. A weak signature of the anomaly can be seen in the pressure-volume curve, which gets enhanced in the universal equation of state, along with that of K-point ETTs. We attribute the change of slope in the universal equation of state near 10 GPa pressure, mainly to hybridization effects. The temperature effect in fact enhances the possibility of L-point ETT. We find that the L-point ETT is very sensitive to exchange correlation terms, and hence we suggest that further refinements in the theoretical techniques are needed to resolve the controversies on the ETT in Zn.

We have carried out electronic structure total energy calculations on rhenium in the hexagonal close packed (hcp) and body centred cubic (bcc) phases, by the full potential linear muffin-tin orbital method, in order to verify the stability of the ambient pressure hep phase against transition to the bcc structure at high pressures. As per our results, no hcp to bcc structural transition can occur up to 1 TPa pressures. Moreover, our Bain path calculations show that face centred cubic and body centred tetragonal structures are also not energetically preferred over hcp in this pressure range. The axial ratio (c/a) of Re changes by less than 0–33% in the pressure range studied.

Results of ab initio electronic structure calculations on the compound, MgB2, using the FPLAPW method employing GGA for the exchange-correlation energy are presented. Total energy minimization enables us to estimate the equilibrium volume, c/a ratio and the bulk modulus, all of which are in excellent agreement with experiment. We obtain the mass enhancement parameter by using our calculated, D(EF) and the experimental specific heat data. The Tc is found to be 37 K. We use a parametrized description of the cal- culated band structure to obtain the T = 0 K values of the London penetration depth and the superconducting coherence length. The penetration depth calculated by us is too small and the coherence length too large as compared to the experimentally determined values of these quantities. This indicates the limitations of a the- ory that relies only on electronic structure calculations in describing the superconducting state in this mate - rial and implies that impurity effects as well as mass renormalization effects need to be included.

Effects of non-local thermodynamic equilibrium (non-LTE) condition on emission and hydrodynamics of typical inertial confinement fusion (ICF) plasmas are studied. The average degree of ionization at high temperatures is seen to be much lower compared to the values obtained from Thomas-Fermi scaling or Saha equation for high-Z element like gold. LTE and non-LTE predictions for emitted radiation from laser-driven gold foil are compared with the experimental results and it is seen that non-LTE simulations show a marked improvement over LTE results. The effects of one group and multigroup, LTE and non-LTE approximations of radiation transport on hydrodynamic parameters are studied for laser-driven aluminium and gold foils. It is further seen that non-LTE and multigroup effects play an important role in predicting conversion efficiency of laser light to X-rays.

Results of ab initio electronic structure calculations on the compound MgB2 using the FPLAPW method employing GGA for the exchange-correlation energy are presented. Total energy minimization enables us to estimate the equilibrium volume, c/a ratio and the bulk modulus, all of which are in excellent agreement with experiment. We obtain the mass enhancement parameter by using our calculated D (E F) and the experimental specific heat data. The T c is found to be 24.7 K.

Details of a two-dimensional X-ray area detector developed using a charge coupled device, a image intensifier and a fibre optic taper are given. The detector system is especially optimized for angle dispersive X-ray diffraction set up using rotating anode generator as X-ray source. The performance of this detector was tested by successfully carrying out powder X-ray diffraction measurements on various materials such as intermetallics AuIn2, AuGa2, high Z material Pd and low Z scatterer adamantane (C10H16) at ambient conditions. Its utility for quick detection of phase transitions at high pressures with diamond anvil cell is demonstrated by reproducing the known pressure induced structural transitions in RbI, KI and a new structural phase transition in AuGa2 above 10 GPa. Various softwares have also been developed to analyze data from this detector.

In the high pressure laboratory at BARC, we are pursuing a program to study the behaviour of materials under static and dynamic pressures. Theoretical component has been an integral part for guiding and interpreting new experiments. The initial phase of such efforts was devoted to the development of equation of state models at arbitrary temperatures and matter densities. With the advent of diamond anvil cell device and the simultaneous provision for laser heating of the compressed samples for static high pressure studies, and with the improvements of the diagnostic techniques in dynamic shock methods, the focus of our studies switched over to the predictions and interpretations of phase transitions. Often these efforts have led to intense experimental studies and sometimes helped in resolving the controversies in data. We adopted the first principles electronic structure calculations for high pressure studies. Our work on the electronic topological transition in zinc led to many experimental and theoretical investigations. The results of electronic structure changes in similar metal cadmium shall be compared with existing understanding in Zn under pressure. Our studies on Nb and other compounds like intermetallics and borocarbides have revealed interesting electronic structure changes under pressure. However, the electronic structure based investigations of structural stabilities at high pressures involve tedious trial and error effort, which is avoided in theab initio molecular dynamics simulations. The current status of our efforts in the use of this technique is illustrated with the example of quasicrystal based clusters.

The method of first principle pseudopotential is used to simulate the volume variation of nuclear Gruneisen parameter. The elements Al and Be, for which Neal’s experimental data exist are investigated. The nuclear Gruneisen obtained from the details of phonon frequencies are in better agreement with the experimental data as compared to those obtained from approximate Slater and Dugdale-MacDonald methods, often used in shock wave studies.

The systematics of the shock constants in shock velocity-particle velocity relations for metals have been examined by energy band theory methods. The causes of non-linearity of this relation at high pressure are discussed in terms ofs ⇌d electron transfer.

Shock Hugoniot calculations are carried out for lead (Pb) using both solid and liquid state theories. The Hugoniots in solid and liquid cases are in mutual agreement within the experimental uncertainties. However, the shock temperatures are quite different as computed from solid and liquid state theories. This fact can perhaps be used to detect melting along the shock Hugoniot provided the shock temperatures are accurately measured.

Saha’s ionization equation has been solved for highZ elements with the aim of providing input for opacity calculations. Results are presented for two elements, tungsten and uranium. The ionization potentials have been evaluated using the simple Bohr’s formula with suitable effective charges for ions. The reliability of the free electron density, ion concentrations, etc., obtained from the Saha’s equation solutions has been checked by comparing thePT andET computed from them with those given by the Thomas-Fermi-Dirac equation of state. The agreement between the two is good from temperatures above 0.2 keV.

The results of electrical resistance (R), thermoelectric power (TEP) and X-ray diffraction measurements on praseodymium (Pr) and its alloys with thorium under pressure are reported. The maximum inR vsP curve exhibited by Pr persists only in the dhcp phase of PrTh alloy. X-ray measurements confirmed that in the alloys also the maximum inR vsP curve is due to the dhcp → fcc transition. Thus the behaviour of Pr and Pr-Th alloys is different from that of La and its alloys with Ce and Th where the maximum in theR vsP curve is electronic in origin and is exhibited by the dhcp, fcc and dist fcc phases.

A procedure is described for computation of opacities for highZ plasmas. Bound-bound, bound-free, free-free and scattering processes are considered. The inputs for these have been obtained by solving IEEOS form of Saha’s equation. Detailed calculations of opacities have been done for tungsten and uranium up to 10 keV of temperature.